1. Field of the Invention
The present invention relates to a probe for measuring the level of a liquid comprising a conductor of given length made from a material whose resistivity depends on the temperature, provided for mounting on said reservoir so that, over at least a first variable part of its length said conductor is immersed in said liquid and, over the second non-immersed remaining part, said conductor is therefore in a gaseous medium, the variable ratio between said first and second parts being thus linked to the variable level to be measured.
Such a probe is used in particular for measuring the oil level in the crank case of an internal combustion engine for motor vehicles.
2. Description of the Prior Art
A probe of this type is already known in which the conductor is a wire made from a nickel, cobalt and iron alloy, whose resistivity has a high temperature coefficient. With this probe, measurement of the level of the liquid is made by applying a current source to the output terminals of the probe and measuring, after a given time interval, the value of the voltage at the terminals of the probe.
The value of the voltage at the terminals of the probe at this time is representative of the variable level of the liquid for the following reasons:
the value of the voltage at the terminals of the probe at all times is representative of the value of the resistance of the total length of the conductor, for it has a constant current flowing therethrough, PA1 the value of this resistance depends on the temperature distribution along the conductor, for the material from which it is formed has a resistivity with a high temperature coefficient, PA1 the temperature distribution along the conductor may be considered as uniform for the first immersed part and uniform for the second non-immersed part, these two parts, both heated by the passage of the current, being at different temperatures because they are cooled differently by the media in which they are located.
When it is desired to make the measurement rapidly, it is too long to wait until the balance temperature is reached for the immersed part, which is very greatly cooled by the liquid and whose temperature rise is slow. Thus, at the time of measuring the voltage, the temperature at the immersed part develops with a time constant very much greater than the time interval elapsing from the time of application of the current source.
On the other hand, the non-immersed part of the conductor which is in a gaseous medium, is much less cooled and it reaches its balance temperature much more rapidly, with a very short time constant.
This disparity of behavior between the immersed part of the conductor and its non-immersed part results in the non linearity of the variation of the value of the voltage at the measuring time, as a function of the variation of the level of the liquid.